[Upper Left]A visible-light image of the geologically active trailing hemisphere of the Jovian moonIo, taken on March 15 1992, when Io was 414 million miles (666 kilometrs) from Earth. HSTresolves features as small as 150 miles (241 kilometrs) across.

[Upper Right]An ultraviolet light (UV) picture of the same hemisphere show's Io's surface. Regions which look bright In visible light are dark in UV. The most likely explanation is that large areas of Io are covered with a sulfur dioxide frost. Because sulfur dioxide is a strong absorber of UV radiation sulfur dioxide-rich areas are dark in the UV though they are bright in visible light.